A radiation image recording and reproducing method utilizing a stimulable phosphor described is now practically employed. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. After a series of those steps, the radiation image storage panel thus treated is subjected to a step for erasing a radiation image remaining therein, and then is stored for the next radiation image recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with a conventional radiography using a combination of a radiographic film and radiographic intensifying screen. Further, the method is very advantageous from viewpoint of conservation of resource and economic efficiency because the radiation image storage panel can be repeatedly used in the method, while the radiographic film is consumed for each radiographic process in the conventional radiography.
The radiation image storage panel commonly employed in the above-described method has a basic structure comprising a support, a stimulable phosphor layer, and a protective film overlaid in order. The phosphor layer generally comprises a binder and a stimulable phosphor dispersed therein. However, there is also known a phosphor layer comprising agglomerate of a stimulable phosphor without binder. The phosphor layer containing no binder can be formed by deposition process or firing process. Further, there is known a phosphor layer comprising agglomerate of a stimulable phosphor impregnated with a polymer material. A radiation image storage panel having any of the above phosphor layers can be used for the radiation image recording and reproducing method.
As is described above, a film of transparent polymer material is generally placed on the phosphor layer so as to protect the phosphor layer. The protective film is generally formed by following two conventional methods. The first method comprises the steps of applying a resin solution onto the surface of the phosphor layer and drying the coated solution (coating method). The second method comprises placing a beforehand prepared transparent resin film on the surface of the phosphor layer via an adhesive layer (film adhering method). Although the coating method comprises a relatively simple procedure and gives the film having strong adhesion, the formed film has such insufficient physical strength that the surface of the coated protective layer sometimes suffers abrasion. On the other hand, the film adhering method comprises a relatively complicated procedure, which generally comprises the steps of beforehand preparing a transparent resin film having an adhesive layer and placing the film on the surface of the phosphor layer. The conventional protective film formed by either method has an advantageous feature in that the protective layer is liable to be stained when the radiation image storage panel having the protective film is conveyed in an apparatus for the radiation image recording and reproducing method, and further it is difficult to remove thus formed stains.
The radiation image storage panel is repeatedly used in the cyclic procedure comprising the steps of exposing to a radiation (for recording of a radiation image), irradiating with stimulating rays (for reading of the recorded image), and exposing to an erasing light (for erasing the remaining image). In this procedure, the panel is transferred from one stage to another stage by means of conveying means such as belt and rolls in the apparatus, and after a cycle of steps is conducted, the panel is piled up on other panel and stored for next cycle. Since the surface of the panel is directly touched to the conveying means (e.g., belt and rolls) during conveying, stains and abrasions are sometimes produced. The stains and the abrasions thus produced on the protective film disturb passages of the stimulating ray and the stimulated emission, and consequently make the resultant image quality poor.
In order to solve the above problem, several improvements are proposed. In copending U.S. patent application Ser. No. 08/469,761 and U.S. Pat. No. 5,227,253, for example, improved protective films are proposed. The proposed films are prepared by applying a coating solution containing a fluororesin onto the surface of the phosphor layer and dried to form a fluororesin-contained protective film. It is stated that the stains and abrasions are hardly produced on the surface of the protective film thus prepared, and further the stains on the surface are easy to remove.
Although the above-mentioned prior arts solve the problem concerning the stains on the protective film, they cannot satisfactorily inhibit occurrence of the abrasions. U.S. patent application Ser. No. 08/834,772 discloses a method for preparing a protective film free from both stains and the abrasions. The method comprises the steps of placing a hard plastic film on the surface of the phosphor layer and then forming a coated fluororesin layer on the surface of the plastic film, to prepare a protective film comprising the plastic film and the coated fluororesin layer provided thereon.
The above-mentioned method effectively keeps the protective film from both stains and abrasions, and therefore a protective film can be advantageously prepared in accordance with the above method in a process for the preparation of radiation image storage panel. However, the inventor has found the problem of the above method: that is, it is not always easy to evenly form a coated fluororesin layer on the surface of the plastic film fixed on the phosphor layer with adhesive. The reason of this trouble appears as follows. Since the films provided on the phosphor layer are required to have a small total thickness so as to obtain a resultant image of a high quality, a very thin film is generally used as the protective film. Consequently, the protective film is so thin that roughness of the phosphor layer surface appears on the protective film surface, and the roughness sometimes further appears on the fluororesin layer coated on the plastic film surface.
Moreover, it is not easy to form a thin coated fluororesin layer on the surface of a multi-layer sheet comprising a support, a stimulable phosphor layer and a plastic film. Although this trouble can be avoided by a method comprising the steps of beforehand forming a coated fluororesin layer on the plastic film and then sticking the plastic film with adhesive onto the surface of the phosphor layer, it is not easy to prepare a thin plastic film having both a coated fluororesin layer provided on one surface and an adhesive layer provided on the other surface.